The initial step in processing rubber is the breakdown of the crude rubber into a soft, plastic state. This step, which is called the mastication step, is complicated by the fact that crude rubber is generally very high in viscosity and, therefore, is difficult to break down. In addition, the high viscosity makes it difficult to admix the many additives into the rubber during processing.
Typically, the crude rubber is placed into a mixer, such as a two-roll rubber mill or a Banbury, to perform the mastication step. During this step, the rubber is mixed with a peptizer (plasticizer) for several minutes under heat until the rubber is sufficiently softened. Then the rubber is dumped, rolled into sheets on a roll mill or a calendar machine and cooled. The rubber is then ready for further processing.
It has been known to employ an alkylaromatic sulfonic acid, e.g., dodecylbenzene sulfonic acid, as a peptizer for natural and synthetic rubbers. The use of sulfonic acid peptizers has been complicated, however, by the fact that, unless care is taken, the alkylaromatic sulfonic acid can cause excessive breakdown of the rubber, leading to a brittle product. In addition, the alkylaryl sulfonic acid is not always fully compatible with the rubber being processed.
The novel processing aid of this invention is a modified alkylbenzene sulfonic acid having improved compatibility with rubber and a more moderate peptizing action when compared with the alkylbenzene sulfonic acid alone.
The novel processing aid of the present invention can be used to reduce and/or eliminate the need for the mastication step. It can be added to the crude rubber in the mixing step and will facilitate the chemical and mechanical breakdown of the rubber, thereby reducing the need for a subsequent mastication step or the peptizer used therein.
The novel processing aid of the present invention has additional benefits as well. When added to crude rubber, it reduces the mixing time required to disperse or emulsify additives into the crude rubber. It also imparts improved physical properties to the polymer, such as improved flow, extrusion and mixing characteristics. Therefore, the composition of this invention is useful in the additive compounding step for rubber, whether the rubber requires premastication (natural rubber) or not (synthetic rubber).
Furthermore, the processing aid of the present invention improves the wetting properties of the rubber. Conventional peptizers, while they do reduce the viscosity of the rubber, do very little to improve the wetting characteristics of the rubber.
It is, therefore, an object of this invention to provide a novel processing aid that can be added to crude rubber during the mixing step which reduces and/or eliminates the need for the mastication step or the peptizer used therein.
It is a further object of this invention to provide a novel processing aid that reduces the mixing time needed to disperse additives into the rubber, that improves the physical properties of the rubber, and that enhances the rubber's wetting characteristics.